In recent years, in the context of efficient use of energy, there have been increasing demands mainly transformer manufacturers and the like, for an electrical steel sheet with high flux density and low iron loss.
The flux density can be improved by making crystal orientations of the electrical steel sheet in accord with the Goss orientation. JP 4123679 B2, for example, discloses a method of producing a grain-oriented electrical steel sheet having a flux density B8 exceeding 1.97 T.
On the other hand, iron loss properties may be improved by increased purity of the material, high orientation, reduced sheet thickness, addition of Si and Al, and magnetic domain refining (for example, see “Recent progress in soft magnetic steels,” 155th/156th Nishiyama Memorial Technical Seminar, The Iron and Steel Institute of Japan, Feb. 10, 1995). Iron loss properties, however, tend to worsen as the flux density B8 is higher, in general.
It is known, for example, that when the crystal orientations are in accordance with the Goss orientation to improve the flux density B8, the magnetostatic energy decreases and, therefore, the magnetic domain width widens, causing eddy current loss to rise.
In view of this, as a method for reducing eddy current loss, some techniques have been used to refine magnetic domains by improving film tension (for example, see JP H02-8027 B2) and by applying thermal strain.
With the method of improving film tension as disclosed in JP H02-8027 B2, however, the strain applied near a elastic region is small, which places a limit on the iron loss reduction effect.
On the other hand, magnetic domain refining by application of thermal strain is performed using plasma flame irradiation, laser irradiation, electron beam irradiation and the like.
For example, JP H07-65106 B2 discloses a method of producing an electrical steel sheet having a reduced iron loss W17/50 of below 0.8 W/kg due to electron beam irradiation. It can be seen from JP H07-65106 B2 that electron beam irradiation is extremely useful in reducing iron loss.
In addition, JP H03-13293 B2 discloses a method of reducing iron loss by applying laser irradiation to a steel sheet.
Meanwhile, it is known that irradiation with a plasma flame, laser, an electron beam and the like increases hysteresis loss, while causing magnetic domain refinement which reduces eddy current loss.
For example, JP 4344264 B2 states that any hardening region caused in a steel sheet through laser irradiation and the like hinders domain wall displacement so as to increase hysteresis loss.
To solve the aforementioned problem, JP 4344264 B2 discloses a technique to further reduce iron loss by adjusting the laser output and the spot diameter ratio to thereby reduce the size of a region, which hardens with laser irradiation in a direction perpendicular to the laser scanning direction, to 0.6 mm or less, and by suppressing an increase in hysteresis loss due to the irradiation.
Furthermore, there has been an increasing demand for a recent transformer to be reduced in noise, as well as to have high flux density and low iron loss, to offer good living conditions. It is believed that the noise of a transformer is primarily caused by stretching movement of the crystal lattice of the iron core, and many studies have shown that reducing single sheet magnetic strain is effective in suppressing the transformer noise (for example, see JP 3500103 B2).
While the method of reducing iron loss disclosed in JP 4344264 B2 reduces the size of a hardening region in the steel sheet, the region with the reduced size of 0.6 mm or less is inevitably formed with an excessively hardening region, which is defined to “exhibit an increase in hardness of the steel sheet surface due to work hardening by 5 or more when measured using a micro Vickers hardness meter.”
Generally, in carrying out magnetic domain refining by irradiating a steel sheet with a heat beam, a light beam, or a particle beam, it is believed that a larger rapid thermal deformation of the steel sheet caused near the irradiated portion and/or a lager reaction force applied to the steel sheet as induced by rapid vaporization of the coating result in a dislocation region with a higher density formed in the steel sheet near the irradiated portion, leading to an increase in the hardness of the steel sheet. We believe that a higher dislocation density increases hysteresis loss, as reported in, for example, “J. appl. phys, 91” (2002), p. 7854 (NPL 2), that a steel sheet exhibits an increase in hysteresis loss upon tensile deformation.
In addition, as the irradiated portion hardens more, the irradiated material tends to suffer more pronounced deflection concaved on the irradiated surface side. The presumed reason therefore is that a higher degree of hardening causes a larger residual stress.
The electrical steel sheets on which magnetic domain refining has been performed by irradiating with a heat beam, a light beam, or a particle beam, are primarily stacked in a flat configuration for use as iron cores of a transformer. A steel sheet with a larger deflection is to be applied with a higher internal stress when flattened for shape correction. Consequently, upon excitation, iron cores take such a deformation mode as to release the internal stress, as well as the deformation due to stretching movement of the crystal lattice, leading to an increase in noise.
Focusing on the fact that the conventional method of reducing iron loss as disclosed in JP 4344264 B2 can reduce the size of a hardening region in the steel sheet, but the region with the reduced size of 0.6 mm or less is formed with an excessively hardening region, which is defined to “exhibit an increase in hardness due to work hardening by 5% or more when measuring the hardness of the steel sheet surface using a micro Vickers hardness meter,” we assumed that the hysteresis loss and noise may be further reduced if such hardening can be suppressed.
Note that it is generally possible to inhibit an increase in hysteresis loss and even an increase in noise if an increase in hardness can be minimized by reducing energy with which the steel sheet is irradiated. When the irradiation energy is reduced, however, there arises a problem that magnetic domain refining becomes less effective in reducing eddy current loss, resulting in a higher total iron loss (hysteresis loss plus eddy current loss) (see “Journal of Magnetics Society of Japan, Vol, 25” (2001), p. 895 (NPL 3)).